This invention relates to the removal of pollutants from gases and, more particularly, to an adsorber system in which pollutant laden gases are passed through columns of activated char to adsorb the sulfur oxides from the gases into the char pellets.
The technique of adsorption to remove sulfur oxides from pollutant-laden gases, such as flue gases from a fossil fuel boiler, is well known. According to this basic process, an adsorber is provided which receives the gases and includes a bed of activated char located in the path of the gases. As a result, the SO.sub.2, oxygen and water vapor in the gases come into contact with, and are adsorbed by, the char pellets. The SO.sub.2 in the gases is oxidized to SO.sub.3 and subsequently catalytically converted to sulfuric acid which is held in the interior pore system of each char pellet. The substantially pollutant-free gases are then passed to a stack, or the like, for exhaustion into the atmosphere.
The adsorber usually contains one or more vertical beds of activated char which moves downwardly in mass flow while adsorbing the pollutants from the gases. The upper portion of the char bed is continuously replenished while the saturated char is collected at the bottom and sent by conveyors to other sections of the system. For example, the saturated char can be sent to a regenerator for regenerating the saturated char by thermal or wet regeneration to reverse the reactions taking place in the adsorber and produce a concentrated steam of SO.sub.2, H.sub.2 O, CO.sub.2, and N.sub.2. The stream of SO.sub.2 can then be further treated to produce relatively pure sulfur.
In these type of arrangements it has been discovered that due to the length of the vertical beds, less adsorption of the pollutants from the gases passing through the lower portion of the adsorber housing is achieved when compared to the gases passing through the upper portion of the housing. This is due to the fact that the char in the lower portion of the housing is less active than that in the upper, since the former is more saturated with pollutants by virtue of its increased exposure to the gases as it passes downwardly in the housing. As a result, it is impossible to achieve a constant average degree of adsorption of the pollutants and, in addition, a less than optimum system efficiency is achieved.
U.S. Pat. No. 3,717,976, of which the present inventor is a co-inventor, teaches a process in which the gases in the lower part of the adsorber are passed through an additional bed of activated char to remove additional pollutants therefrom. However, this method is limited to the use of only one bed of char extending through the upper and lower portions of the adsorber and one additional bed of char in the lower portion of the adsorber. As a result, the adsorption capacity of the adsorber is severely limited.